This invention relates to highly efficient processes for the preparation of tetrabromobisphenol-A.
Tetrabromobisphenol-A is one of the most widely used brorninated flame retardants in the world. It is used extensively to provide flame retardency for styrenic thermoplastics and for some thermoset resins.
The commercial processes used to produce tetrabromobisphenol-A generally fall into three categories. The first category includes those processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 40-50 pounds of methyl bromide can be expected per 100 pounds of tetrabromobisphenol-A produced. The methyl bromide co-production is now considered desirable since there is a substantial market for this bromide as a fumigant and as a pharmaceutical or agricultural chemical intermediate. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ar-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ar-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feeds are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See U.S. Pat. No. 4,990,321; U.S. Pat. No. 5,008,469; U.S. Pat. No. 5,059,726; and U.S. Pat. No. 5,138,103. Generally, these processes brominate the bisphenol-A at a low temperature, say 0 to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause the precipitation of the tetrabromobisphenol-A from the reaction mass. Additional water for that purpose is added at the end of the reaction. One drawback with this type of process is that it uses a fairly long aging or cook period after the reactants have all been fed and it requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See U.S. Pat. No. 3,929,907; U.S. Pat. No. 4,180,684; U.S. Pat. No. 5,068,463 and Japanese 77034620 B4 77/09/05. The solvent is generally a water immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenol-A is found in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. This process type is not a panacea though, as there is the expense of handling, purifying and recycling the halogenated organic solvent. In addition, the cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
As long as there is a viable market for methyl bromide, the processes of the first category have been found to be commercially attractive. However, it is now being proposed, on an international level, that the use of methyl bromide as a fumigant be prohibited. Since the fumigant market is the main market for methyl bromide, a need is apparent for tetrabromobisphenol-A processes which do not co-produce a substantial amount of methyl bromide. This is a difficult task since such processes, to be commercially successful, will be required to economically produce tetrabromobisphenol-A without the benefit of the revenue realized from the sale of co-produced methyl bromide.